During the design phase, Mr. Leonard said a series of vaults was discovered along the street underneath the sidewalk.

"One is an old coal drop with circular cast-iron covers that open up," he said. "You can see the basement. They're old construction made from stone and granite, not poured concrete."

The problem is, the city can't do anything with them, Mr. Leonard said, without compromising the structural integrity of the units — and the sidewalks above.

"The vault ceiling is also the sidewalk and we need to keep the thickness to make sure we maintain public safety," he explained. "You can't add or take away from the thickness of the material. It's a great historical feature we had to incorporate into the design."

He said the vaults have a lot more history to them than people think.

Senior Engineer Paul M. Furgal with Tighe & Bond, who is also project manager for the project, said old vaults are quite common in older communities that have downtown areas.

"They used to rely on coal deliveries for the building's heating," he said. "Boston has a lot of them in the financial district. You'll see no parking signs because there is a hollow vault below."

Many were used to store mechanical equipment, he said.

"It is something I've certainly dealt with," he said. "It is a difficult challenge to improve certain sidewalks. It puts a hurdle there when you're looking at a systemwide improvement that is contiguous."

Department of Public Works Commissioner Lenny Laakso said the old Johnsonia Building on Main Street that was destroyed by fire in 2011 also had an underground vault.

"There were glass circles in the sidewalk to let light into the basement," Mr. Laakso said. "I remember as a kid walking up the sidewalk and seeing glass circles."

At Raymond Apartments on Day Street (formerly a hotel), the city worked with the owner to repair the sidewalk because the building's vault holding up the sidewalk had deteriorated, he said.

John Spokis, assistant civil engineer, for Worcester DPW, said the city is also dealing with underground vaults during improvements downtown.

Some run the length of a building, he said.

"They could have been used as offices or a lunchroom or a bathroom — there is always a surprise when you go down," Mr. Spokis said. "There could be a safe. Every time you go into a vault, there could be something different."

On Main Street, he said there are a few with steel doors that open up to a conveyor used as a coal chute.

"They can't be moved," he said. "Sometimes we've asked the owners of the buildings to build walls so we can fill it in with gravel and finish off the sidewalk."

He said it is possible there could be some hidden vaults under the city they do not know about.

"You hear talk about catacombs from the 1700s and 1800s that run from some of the basements all the way to the Blackstone River to the ships," he said. "It is some of the stuff you hear over the years. We don't know where they are, but the stories you hear is that they could have been used for shanghaiing sailors or illegal trade."

Contact Paula Owen at powen@telegram.com. Follow her on Twitter @PaulaOwenTG.

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